Many organic amines in their purest forms are typically water-white, liquid substances in appearance. However, owing to the presence of one or more impurities present or formed during the manufacturing or processing of amines, amine products are well-known to often have the tendency to take on color over the course of time, for example, by virtue of their having been contained in a storage tank or other containment vessel. Typically, amines turn brown with the passage of time.
In general, amines can be decolorized to become a color-stable product by hydrogenation, or by treatment with activated carbon or acid, followed by distillation. However, the extra processing steps so required result in yield loss, and add overall cost to a product so treated. Other methods for decolorizing or otherwise providing color-stable amine products have been described by various workers as well.
For example, U.S. Pat. No. 5,663,444 teaches a process for the preparation of a color-stable dialkylaminoethanol of the formula:
in which R1 and R2 independently are C1–C20-alkyl, by reacting ethylene oxide with a dialkylamine of the formula HNR1R2, in which R1 and R2 have the same meaning, in the presence of from 2.5 to 50% by weight of water, based on the reaction mixture, at a temperature of from 95° to 170° C., and separating off the water and high-boiling constituents by distillation under a reduced pressure and at a temperature of from 40° to 90° C. at the column bottom.
U.S. Pat. No. 4,567,303 discloses a process for preparing colorless alkanolamines and/or alkylenediamines which comprises thermally reacting ammonia (or a reactive amine and an alkylene oxide or alkanolamine) in a reactor in which the corrosion-sensitive parts of the reactor are made of substantially nickel-free stainless steel.
U.S. Pat. No. 3,567,779 provides a method for stabilizing amines against color change at reflux boiling temperatures by incorporating in the amine a stabilizing amount of a mono- or di-lower alkanolamine.
U.S. Pat. No. 4,766,247 discloses a process for the reduction of the color of polyamines by reacting at elevated temperature, i.e. 50° to 175° C., and pressure, i.e. 50 to 1500 psig, such colored polyamines, e.g. triethylenetetramine or tetraethylenepentamine, in the presence of a hydrogenation catalyst, e.g. Raney nickel or palladium on carbon, and a hydrogen atmosphere for a period of time sufficient to effectuate the desired reduction in color. The polyamines can either be distilled into a narrow product composition and then hydrogenated or a crude polyamine product can be hydrogenated and then distilled to produce the desired product composition.
U.S. Pat. No. 4,731,165 teaches a process of de-coloring crude triethylenetetramine (TETA) is disclosed. In the disclosed and preferred procedure, a sulfonic acid ion exchange resin acts on the crude TETA to enable subsequent distillation at elevated temperature to obtain de-colored TETA.
European Patent EP 0 028 555 teaches the purification of dialkylaminoethanols by subjecting them to catalytic hydrogenation treatment in the heterogeneous phase prior to distillation.